Nucleating agent (NA) species with solubility and self-assembly ability can readily and effectively manipulate the crystalline morphology of semicrystalline polymers through construction of heterogeneous frameworks prior to primary crystallization of basal resins. However, the solubility of NA is difficult to be assessable by current traditional methods. In this study, gradient temperature field (g-T field) was utilized to ascertain the dissolution and self-assembly behaviors of beta-NA in the melts of isotactic polypropylene (iPP). The g-T field technique can facilely assess the soluble behavior of beta-NA through determining the transformation between several NA frameworks, namely, needle-, flower- and dendrite-like supermolecular structures. Clarifying the soluble behavior of beta-NA is of great significance to guide the formation of various crystalline frameworks under the homo-temperature fields and control the resultant crystalline morphology of beta-modification iPP. Some interesting findings are summarized as: (1) an in-situ observation under g-T field clearly indicates the sequential occurrence of various nucleation and crystallization events on the same observed window, and approves for the migration of well molten beta-NA, (2) the exact correlation between Tf and framework type reveals that an abrupt transformation (over narrow temperature range of 1oC) occurred between needle-like and dendrite-like frameworks, (3) the primary crystallization of iPP is strongly depending on the construction mode of beta-NA framework.